--- a/browser/installer/Makefile.in
+++ b/browser/installer/Makefile.in
@@ -142,16 +142,24 @@ DEFINES += -DMOZ_JEMALLOC3
 endif
 DEFINES += -DMOZ_ICU_DBG_SUFFIX=$(MOZ_ICU_DBG_SUFFIX)
 ifdef CLANG_CXX
 DEFINES += -DCLANG_CXX
 endif
 ifdef CLANG_CL
 DEFINES += -DCLANG_CL
 endif
+ifeq (x86,$(CPU_ARCH))
+ifdef _MSC_VER
+ifndef CLANG_CL
+DEFINES += -DWOW_HELPER
+endif
+endif
+endif
+
 
 libs::
 	$(MAKE) -C $(DEPTH)/browser/locales langpack
 
 ifeq (WINNT,$(OS_ARCH))
 PKGCOMP_FIND_OPTS =
 else
 PKGCOMP_FIND_OPTS = -L

--- a/browser/installer/package-manifest.in
+++ b/browser/installer/package-manifest.in
@@ -778,16 +778,19 @@
 @RESPATH@/components/pipboot.xpt
 @RESPATH@/components/pipnss.xpt
 @RESPATH@/components/pippki.xpt
 
 ; For process sandboxing
 #if defined(MOZ_SANDBOX)
 #if defined(XP_WIN)
 @BINPATH@/@DLL_PREFIX@sandboxbroker@DLL_SUFFIX@
+#if defined(WOW_HELPER)
+@BINPATH@/wow_helper.exe
+#endif
 #endif
 #endif
 
 ; for Solaris SPARC
 #ifdef SOLARIS
 bin/libfreebl_32fpu_3.so
 bin/libfreebl_32int_3.so
 bin/libfreebl_32int64_3.so

--- a/security/sandbox/moz.build
+++ b/security/sandbox/moz.build
@@ -13,16 +13,20 @@ elif CONFIG['OS_ARCH'] == 'WINNT':
     FORCE_STATIC_LIB = True
 
     DIRS += [
         'staticruntime',
         'win/src/sandboxbroker',
         'win/src/sandboxtarget',
     ]
 
+    if (CONFIG['CPU_ARCH'] == 'x86' and CONFIG['_MSC_VER'] and not
+            CONFIG['CLANG_CL']):
+        DIRS += ['win/wow_helper']
+
     EXPORTS.mozilla.sandboxing += [
         'win/src/logging/loggingCallbacks.h',
         'win/src/logging/loggingTypes.h',
         'win/src/logging/sandboxLogging.h',
     ]
 
     include('objs.mozbuild')
     SOURCES += security_sandbox_cppsrcs

new file mode 100644
--- /dev/null
+++ b/security/sandbox/win/wow_helper/Makefile.in
@@ -0,0 +1,42 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# We need to build a 64-bits binary during a 32-bits build. This requires
+# a different compiler and different library paths. Until the build system
+# supports this natively.
+
+# Some Make magic to avoid CXX and LIB being evaluated when nothing
+# is built in this directory
+lazy = $(if $(___$(1)),,$(eval ___$(1) := $(2)))$(___$(1))
+
+# We could use the `which` python module, but it needs more code to handle
+# the situation where CXX points to an absolute path. But using the shell
+# which returns a msys path, while we need a windows path. So force msys
+# to do the conversion for us by calling python with an environment variable
+# with the result of the call to `which`. Then munge that path to add the
+# x64 cross-compiler path.
+ifdef MOZ_USING_COMPILER_WRAPPER
+ORIG_CXX := cl
+else
+ORIG_CXX := $(CXX)
+endif
+CXX = $(call lazy,CXX,"$$(subst amd64_x86/x86_amd64/,amd64/,$$(shell CL=`which "$(ORIG_CXX)"` $(PYTHON) -c 'import os; print os.path.dirname(os.environ["CL"])')/x86_amd64/cl.exe)")
+
+MOZ_WINCONSOLE = 0
+
+include $(topsrcdir)/config/config.mk
+
+# Munge the LIB variable to contain paths to the x64 CRT and system libraries.
+# Unconveniently, none of the paths have the same convention, including the
+# compiler path above.
+LIB = $(call lazy,LIB,$$(shell python -c 'import os; print ";".join(s.lower().replace(os.sep, "/").replace("/vc/lib", "/vc/lib/amd64").replace("/um/x86", "/um/x64") for s in os.environ["LIB"].split(";"))'))
+
+CXXFLAGS := $(filter-out -arch:IA32,$(CXXFLAGS))
+
+# OS_COMPILE_CXXFLAGS includes mozilla-config.h, which contains x86-specific
+# defines breaking the build.
+OS_COMPILE_CXXFLAGS :=
+
+# LNK1246: '/SAFESEH' not compatible with 'x64' target machine
+LDFLAGS := $(filter-out -SAFESEH,$(LDFLAGS))

new file mode 100644
--- /dev/null
+++ b/security/sandbox/win/wow_helper/moz.build
@@ -0,0 +1,30 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Program('wow_helper')
+
+SOURCES += [ '../../chromium/sandbox/win/wow_helper/' + f for f in (
+    'service64_resolver.cc',
+    'target_code.cc',
+    'wow_helper.cc',
+)]
+
+LOCAL_INCLUDES += [
+    '../../',
+    '../../../',
+    '../../chromium/',
+]
+
+DISABLE_STL_WRAPPING = True
+
+DEFINES['UNICODE'] = True
+
+USE_STATIC_LIBS = True
+
+# The rules in Makefile.in only force the use of the 64-bits compiler, not
+# the 64-bits linker, and the 32-bits linker can't do 64-bits compilation for
+# PGO, so disable PGO, which is not interesting for this small binary anyways.
+NO_PGO = True